Gas turbine engines generally include a gas generator which comprises a compressor for compressing air flowing aft through the engine, a combustor in which fuel is mixed with the compressed air and ignited to form a high energy gas stream, and a turbine driven by the gas stream and connected for driving a rotor which in turn drives the compressor. Many engines further include a second turbine, known as a power turbine, located aft of the gas generator and which extracts energy from the gas stream to drive a rotating load with variable pitch blades such as found in the propulsor of helicopters, ducted turbo-fan engines, and turbo-prop engines.
A recent improvement over the turbo-fan and turboprop engines described above is the unducted fan engine such as disclosed in U.S. patent application Ser. No. 437,923--Johnson, filed Nov. 1, 1982. In the unducted fan engine, the power turbine includes counterrotating rotors and turbine blades which drive counterrotating unducted fan blades radially located with respect to the power turbines.
The fan blades of the unducted fan engine are variable pitched blades to achieve optimum performance. During operation, fuel efficiency of the engine can be increased by varying the pitch of the blade to correspond to specific operating conditions.
In one prior art apparatus, when it is desired to change the pitch of fan blades coupled to a rotating member which are concentrically disposed about a stationary member, the pitch is varied by a bearing arrangement coupled to a gearing arrangement. One such mechanism is described in U.S. patent application Ser. No. 647,283 filed Sept. 4, 1984 now U.S. Pat. No. 4,657,484. There, the pitch of fan blades is varied by a hydraulic actuator mounted inside the static power turbine support structure. The motion from the actuator is first transmitted to the rotating member by a system of bearings and then to the blades by a system of gears and linkages mounted on the rotating member. It is believed that one disadvantage of using such a pitch change mechanism is the weight of the mechanism. A large actuation force is required to vary the pitch of the propulsor blade as well as to maintain a particular blade pitch when the engine is producing a thrust. Since the actuation force must be transferred to the blade by way of the bearings, gears and linkages, the bearings, gears and linkages must be sufficiently sturdy to transfer the forces without substantially deflecting or deforming. Any deflection or deformation of the mechanism may cause play in the system and, consequently, cause the fan blades to flutter slightly as they are rotated and may also cause a torsional imbalance on the engine. In order for the mechanism to be sufficiently sturdy, it must be massive. The weight of the mechanism added to the rotating member may detrimentally affect the efficiency of the system by increasing the inertia required to turn the rotating member. It is believed another disadvantage of the prior art blade pitch varying mechanism is the poor accessibility of the mechanism. A large portion of the mechanism is built into the stationary power turbine structure. In order to access this portion, one must penetrate the power turbine. Consequently, the location of the mechanism makes access and maintainability quite difficult. It is believed that a further disadvantage of the prior art blade pitch mechanism is poor wearability of the mechanism. The mechanism employs a plurality of racks connected to a corresponding plurality of pinion gears. Positioning of the pinion gears by the racks varies the pitch of the blades. For any given rack and pinion, only a few gear teeth of the pinion will intermesh with a few gear teeth on the rack. Accordingly, the entire force required to maintain a particular blade pitch is carried by these intermeshing teeth. During normal flight and normal operation of the engine, the blade pitch angle will remain relatively constant. Thus, the few intermeshing teeth may wear and/or fail whereas the other teeth may have very little wear.